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Mdoc
Ehh... munch munch... what's up, Doc?
Premium Member
join:2007-03-27
Sterling, VA

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Mdoc

Premium Member

Q about engine running with dead or disconnected battery

Got a question for any mechanic here.
I remember years ago driving my car with a dying battery, and as a result the engine would tend to stop running if there was not enough current for the ignition coil to keep it running, due to all lights turned on. That was my experience.

Now there's a guy in another forum saying the engine will continue to run when the battery is disconnected. I understand it will run, but what's the truth if the lights are turned on? I think that whether the engine runs (with the battery disconnected) depends on the load on the alternator and (I guess) it also depends on the engine design itself, but what's the truth?

neonhomer
Dearborn 5-2750
Premium Member
join:2004-01-27
Edgewater, FL

neonhomer

Premium Member

Used to be, on older cars, you could run the vehicle w/o a battery. The alternator would generate enough power to keep the engine running. Also, old carb'd vehicles didn't require a lot of juice to run, so you could do it on alt alone.

Newer vehicles requires a constant 12V+ signal from the battery to the PCM to stay running.

Cho Baka
MVM
join:2000-11-23
there

Cho Baka to Mdoc

MVM

to Mdoc
Assuming that we all know it isn't a good idea due to the potential for voltage spikes...

Generally, the engine should run with the battery disconnected.
If electrical loads exceed the capacity of the alternator at any given engine speed, the voltage will drop and the engine will stall.

As there is no battery, if a heavy load is switched on and the alternator doesn't react quick enough, then the voltage will drop and the engine will stall.
(Obviously the opposite is true as well, if a heavy load is dumped, then the voltage will spike well higher than 14 V - creating the potential for damage.)
Part of the role of a battery is to prevent these two scenarios.

I've had a couple cars that got towed in, I boosted them and drove them into the shop. The brake lights were enough to make the car stall. The cause in these cases was a frozen battery that had cracked and leaked out all the electrolyte from the cells on the ends. (0V) (Discharged battery and cold weather = frozen battery.)

In addition to these 2 basic scenarios, there are a multitude of variables on newer cars.
Examples:
Charging systems controlled according to battery charge level and temperature.
Vehicle electrical systems that dump non-critical loads if voltage or charge levels are dangerously low.

Mdoc
Ehh... munch munch... what's up, Doc?
Premium Member
join:2007-03-27
Sterling, VA

Mdoc to neonhomer

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to neonhomer
said by neonhomer:

Newer vehicles requires a constant 12V+ signal from the battery to the PCM to stay running.

PCM=Powertrain Control Module?
Mdoc

Mdoc to Cho Baka

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to Cho Baka
Thanks guys, that confirms my suspicion.

The Pig
I know you want to be me
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join:2009-09-11

The Pig to Mdoc

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to Mdoc
I have an 1988 and one time the battery died while driving and the engine conked out!

aurgathor
join:2002-12-01
Lynnwood, WA

aurgathor to Mdoc

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to Mdoc
I had a '71 Maverick and a '72 Ford LTD, and they were both capable or running (engines) without a battery. I never actually tried to drive them that way in traffic, however.

But I know for the fact the Maverick would still drive just fine with battery down to 6V or so with a dead alternator.

SmokChsr
Who let the magic smoke out?
Premium Member
join:2006-03-17
Saint Augustine, FL

SmokChsr to Mdoc

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to Mdoc
Typically speaking if the battery is good, once you get the engine started the car will run with the formerly dead battery. Keep in mind that as soon as the alternator starts making electricity some charge is returned to the battery. Putting an amp gauge on the battery lead will likely show a charge current of 50+ amps. It will be back up to operating voltage in just a few seconds. It will still take some time before enough charge has built for it hold for long should the alternator stop providing power.

Battery removed, that's just a good way to cook all your electronics. It will work most of the time, but since the battery also acts as a big fat capacitor smoothing out all the peaks & dips that will occur you run a very high risk of serious damage.

mr_slick
join:2003-05-22
Lynnwood, WA

mr_slick to Mdoc

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i used to drive an old '65 chevy truck without a battery sometimes- i just had to be home before nightfall!

of course that was also the same truck that i had, for a time, ran the coil directly off the battery so when i wanted to shut it off i had a 'friend' get out and pull the wire we all got a jolt out of that!

it had a 400 small block out of a 70s olds but not much else (if anything) demanding electricity. newer vehicles you wouldn't get very far without stalling out.

damn i miss that truck!!!
mr_slick

mr_slick to Mdoc

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we also (in our stupidity and youth and without a multimeter) used to use that method to see if the alternator was good. if you could take off the battery and it kept running then the alternator was good and either the battery was bad or you had poor connections somewhere.

aurgathor
join:2002-12-01
Lynnwood, WA

aurgathor

Member

Older cars that had no active semiconductors in them wouldn't care much, and could take that abuse much better than newer cars.

But that alternator test (unless you turned on the heater and headlights before disconnecting the battery) wouldn't show if the alternator was weak.

TheHarvester
Premium Member
join:2006-08-25
Dana Point, CA

TheHarvester to Mdoc

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***This is all assuming the alternator is in proper working order and the correct one for any specific vehicle.***
There is a big difference between a dead, dieing and missing battery.
For the most part a battery, be it dead or dieing will act as a shock absorber for the electrical system and help prevent damage to sensitive electronics. Now, removing a battery from a running car, you remove the "shock absorber" and the alternator can and will spike possibly damaging sensitive electronics. If everything survives the vehicle should continue to run as long as there is gas in the vehicle. I don't think there is a normal stock vehicle that will have an electrical load that would be larger than the alternators output. After all it (the alternator) needs to supply enough for the electrical load (driving at night in stop and go traffic listening to the radio during a freak summer storm with the A/C running) and charge the battery under normal conditions.

Now, I have seen batteries that were in a certain bad condition where they basically absorb (for lack of a better word) most of the alternators output and not allow enough to supply the electrical load to keep a vehicle running.

mr_slick
join:2003-05-22
Lynnwood, WA

mr_slick to aurgathor

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to aurgathor
i guess not, but it gave an indication of why the battery was always dying = bad/old battery instead of wondering if the alternator was sending a good charge.

we also checked for dimming headlights

and this was true redneck backyard mechanics and i was a wee squirt that learned (and mis-learned from said rednecks)

Mdoc
Ehh... munch munch... what's up, Doc?
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join:2007-03-27
Sterling, VA

Mdoc to TheHarvester

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to TheHarvester
said by TheHarvester:

After all it (the alternator) needs to supply enough for the electrical load (driving at night in stop and go traffic listening to the radio during a freak summer storm with the A/C running) and charge the battery under normal conditions.

Heh heh. Don't forget the running wipers, cruise control, and fog lamps.

TheHarvester
Premium Member
join:2006-08-25
Dana Point, CA

TheHarvester

Premium Member

Well the "freak summer storm" was meant to indicate the wipers and I wouldn't recommend cruise control while it is raining.